In 1996, an email company named Hotmail launched with a small group of users. By the end of 1997, that company had 12 million users.

How did they grow so dramatically in just year and a half? Well, they looked at their initial numbers and saw that approximately 80 percent of new users came by referral from current users. To make the most of referrals, Hotmail created their iconic email postscript: “PS. I love you. Get your free email at Hotmail.”

Read more at Shiptenan.com.